well first of all I will say that the location is not a surprise. dogs are naturally drawn to urinate on elevated surfaces, AND porous surfaces.

however, it sounds like its extremely out of the ordinary for any of them to have done it, so that *might* mean whoever did it may have a bladder infection or something along those lines. OR, it couldve just been a random accident. behavior is constantly changing, and you can never assume anything will be 100% with dogs behavior. lol they surprise us all the time!

my other thought is, if they arent allowed on the bed, why invite them up?? do they truly understand the limits? they may not. dogs are pretty tricky, and often dont learn what we think they have. for example, alki learned not "dont counter surf" but "dont counter surf when mom is present"! lol when dogs are punished for something they often learn that its not "safe" to do it in your presence, and quickly learn to do it when youre not there. so there could be a chance that one or more of them have learned that its not "safe" to get on the bed when youre around because they get punished, so they just do it when no one is around.

my other thought is, how old are each of them? have they had accidents on the bed EVER in the past before they became reliable in the house? and how long have they each been reliably potty trained? potty training is interesting.. dogs dont learn the difference between inside & outside, they learn substrate and location. so for example when successfully potty trained a dog may learn not to potty on carpet, and to potty on grass. the other tricky thing about potty training is it is SELF-reinforcing, meaning, the act of eliminating is reinforcing in itself. so if when young one or more of your dogs had a couple accidents on the bed it may have become enough of a habit or reinforcement that when faced with a situation of not having access to the most reinforcing potty area (backyard) they fall back on one that had past reinforcement, the bed. (Alki had a spot in my old apt that she would potty on when left loose for over 6-8 hrs, but was completely reliable otherwise).

So.. just some things to think about there. If it were me Id just assume that for whatever reason one of your dogs really had to go potty (maybe they hadnt pottied recently before they were let in) and so they just found a place they felt was acceptable to eliminate. Id chalk it up as a random freak thing and not worry about it unless it continues.