When you do pick up and drop offs, your husband should be doing it, with you as witness.
Document what happens. how late are we talking here? if the children eventually get returned, it is a no harm, no foul situation.
Has anything like this even happened yet?
If she has a pattern of being consistently extremly late, your husband may be able to modify visitation based on that. A good thing to think about if you ever modify again, would be to ask that the person getting the child either for visits or back, be the one responsible for the transportation, that way you are not left waiting with the child for someone to show up.-example- you want kids for visitation you go all the way to get them. Visitation over? Custodial parent goes all the way to get them.
other than that, it may be something you just have to work into your schedule. 15 minutes is a very short period of time, to have to wait. Don't obsess over "collecting evidence for court". just document and forget it, and realize there are always going to be little things that bug you. Save your energy for something big, and hopefully it will never happen. RELAX! (and please let your husband take over most of the parenting already!...)
"It is easier to build strong children than repair broken men." Frederick Douglas