Are we safe in ordering drug testing based on a tip?

Q: “One of our employees who was recently terminated mentioned in an exit interview that they know of other employees who have marijuana in their luggage when traveling for the company. Our company requires that employees drive to job sites and stay in a motel for a few days while doing the job. The employees travel in our company cars. As a result of this claim, we would like to do drug tests on all the field employees on a day when they come into the office for open enrollment. Also, we would like to search the vehicles that are leaving town to make sure there are no drugs in the trucks. Can we search the luggage and backpacks that are in our vehicles? Do you have any concerns about drug testing all field employees at one time? We have a drug policy that states that we do drug testing pre-employment, and may do post-accident or randomly if suspicious (we have not been consistent with this)." – Cristy, NC