My bike was moved right across the parking at Loudeac in 2007 - took ages to locate it. Maybe marshals needed to redistribute the weight on the 'crowd control barriers' they use at some controls to lean bikes on? I've seen a whole section start to collapse when unevenly weighted - like if a whole peloton take theirs away from one side! Also someone knocked mine over in 2003 - I came back to find the drop of the bars bent in on one side by about an inch. Not ride-ending but annoying. Ever since, if I can find a wall to lean it on or a bit of grass out of the way to lay it down, then I will.
Villaine la Juhel has wooden 'wheel-benders' that don't fit with wider tyres, so again, find a wall!
I have my mascot on my bar bag, which makes locating the bike among hundreds of others relatively easy, even at night in a dimly lit school playground. It also helps that it's a large frame and Carradice bar bags sit high!