Customs officials in every country have the right to inspect any of the items that you take across the border of the country of their employ. They do not need an excuse, such as "look[ing] suspicious" to implement a search. This may be a key factor in deciding to perform a complete body search, but not for an inspection of a vehicle or personal effects. Travellers crossing border points are routinely subjected to close inspections, principly to determine if they are smuggling or carrying contraband materials. Depending on the number of customs officers and inspection points at a customs point, officials may perform frequent close inspections of travellers. 'Frequent' may simply mean the next time a customs officer has completed one inspection and has become available to perform another. They tend to keep the inspection points occupied at major crossing points. This is not always the case at smaller crossing points, which I like to use, if possible, because the vehicle lineups are shorter and they are less liable to detain you for an inspection. Having passed through customs by road and air many times, though not between the US and Mexico, I have regularly seen vehicles stopped for a detailed inspection. It has happened to me on a few occasions, the first as a child during a vacation with my parents and sister. I have been through close inspections both while entering the US and returning to Canada, where I hold citizenship. Though I have never had a physical search of my body, I have been asked a list of questions to verify that I had local knowledge (e.g., who is the mayor of the city you claim to reside in).