I believe it can happen, and it doesn't necessarily have to be a result of insecurity or lack of understanding. I think its particularly a concern in circumstances where a founder may not possess the necessary skills to build the entire product (hardware especially). The concern may be that by reaching out to experts for help, if the idea is indeed a valuable one, who's to stop the consulted experts from running with it?
I know someone who was subject of this. He had a very cool and quite unique hardware product idea. He needed help with specialized hardware design, so he reached out to an expert (who has many more years experience and much deeper pockets). 10 months later, this expert is developing the same exact product, with the same features, even some of the same marketing content (line for line). There is no way to be certain that this expert took the idea, but it's highly suspected. At least for me, I don't see why it can hurt to put a little effort in to be a bit more careful.