It's the oldest sales trick in the book: Tell customers demand is high and supply low, and if they don't act quickly an offer will disappear. The nonprofit advocacy group Twin Cities Consumers' Checkbook found that many hotel-booking websites take this tactic to extremes.

In a recent investigation, it found that when travelers shop for rooms on many travel-booking websites, search results issue constant false warnings about scarcity.

For example, as you scroll through listed properties at Agoda.com, details about amenities and rooms for some hotels quickly disappear, replaced by "JUST MISSED IT!" and "Sold out on your dates! Our last room here is already booked. Check out these similar properties before they're also sold out."

Among hotels that still have rooms available, almost all get a bright red "Popular!" icon or bold warning of low inventory ("ONLY 1 LEFT!"). And if you're not in panic mode yet, at the bottom of the page a banner pleads "Rooms in [area you've searched] are in high demand. Book yours now!" Booking.com, Agoda's parent site, makes similar breathless calls.

The search results at Hotels.com (owned by Expedia) paint a similar picture of an accommodation drought for most destinations: Many hotels are marked as "Sold Out," and as you scroll down, pop-ups detail the number of people looking at rooms you're considering. Like on Booking.com, many properties get a prominent red alert — "Only X left" at the price shown.

Checkbook's researchers spent weeks searching various hotel booking websites. They encountered such high-pressure sales tactics at Agoda, Booking.com, Expedia, Hotels.com, Hotwire, Orbitz, Priceline and Travelocity. The design of each website constantly indicated that unless the shoppers quit their pesky shopping around and booked right away, they would miss out.

Checkbook found most dire warnings about low availability were dishonest: There's usually still plenty of room left at the inns. Checkbook conducted 80 searches for hotel stays in major cities across these eight websites; all spit out misleading messages. Checkbook repeated these searches three times over three weeks. Each time, shoppers got alerts about shortages that typically didn't exist.

When Checkbook's shoppers searched Agoda for a stay in Las Vegas, it added "ONLY 1 LEFT" next to the Homewood Suites by Hilton listing. But shoppers found that that referred to only one room type — a "1 King accessible." There were also three other room types available at that price, and Checkbook counted at least 19 rooms available for the same rate. The site still proclaimed, "Limited availability" and "This is a popular choice!" next to every option.

One of Checkbook's searches on Priceline was for a spot in downtown Minneapolis. Shoppers received incredibly bad news: For 23 of the first 25 properties shown, the site claimed only one room remained. But when shoppers clicked to book these seemingly near-capacity hotels, they found many rooms available. At the Radisson Red, for instance, dozens of rooms were still up for rent.

So, what gives?

Often, when these websites warn that only one room is left, they're referring to a cherry-picked room type with low availability. For example, Expedia told Checkbook "We have 1 left at $89 per night" for the La Quinta Inn & Suites in downtown Dallas. But after clicking on the listing, shoppers discovered Expedia's warning applied only to a handicapped-accessible room with a king bed.

This was a typical ploy. Checkbook found that most websites' shortage warnings weren't on the level; they merely selected obscure room types with low inventory so they could serve up misleading stats on shortages. While it's technically true that a website had only one remaining handicapped-accessible junior queen suite with breakfast, it's not fair to customers to shout "ONLY 1 LEFT" when more than 25 choices still exist, often for the same price.

But Checkbook also often found that booking sites don't even bother to point to obscure room types to falsely advertise low availability. When shoppers searched for one stay on Orbitz, a listing for the L.A. Grand Hotel Downtown claimed only three rooms were left at $211 per night. But a click on the listing yielded two near-identical room choices for that price, and for these other booking options Orbitz reported nine rooms were available at $211 per night. So, combined, Orbitz listed 12 available rooms for $211, and if shoppers were willing to pay $212 per night, there were even more choices.

Even at hotels where Checkbook found actual low remaining room counts, the "shortages" lasted for weeks. After conducting 80 sample searches, Checkbook's shoppers repeated them a week later, and again two weeks after that. Despite sites cautioning to "Hurry!" or "Book now!" Checkbook found neither availability nor prices suffered with time.

Among sites Checkbook tested, Priceline had the most claims of room shortages. For the 10 searches that Checkbook's shoppers conducted using it, on average 21 of its first 25 listings had warnings about low availability. Orbitz was the next-worst in this department.

These websites clearly work to deter their customers from shopping around. Checkbook concluded that by claiming only one or two rooms remain when the property has plenty, these websites are employing misleading and dishonest sales tactics. Don't let hyped-up language stop you from finding the best room at the best price.

Twin Cities Consumers' Checkbook magazine and Checkbook.org is a nonprofit organization with a mission to help consumers get the best service and lowest prices. We are supported by consumers and take no money from the service providers we evaluate. See Checkbook's full report on this topic, and all of Checkbook's ratings and advice, free of charge until Dec. 5 at Checkbook.org/startribune/hotels.