I just had a breakfast (my fav way to have a 'meeting') with a fellow who offered me a terrific alternate view into a problem I've been chewing over.
Was that the main goal of our breakfast? Did I ask directly for the input? Nope. But it was a a fabulous part of the exchange. He appreciated the bits I brought him and I, in turn, got a nudge in my thinking.
That's informal mentoring at it's best - simply being open to what folks can offer you and not dismissing good information simply because you didn't ask for it directly.
By that definition, informal mentoring can happen all the time.
Of course, it's hard to keep one's mind open. I know mine operates at half-mast most days. So there's another way into informal mentoring that's more deliberate: asking someone you respect/like to coffee and asking their opinion.
Asking someone for their opinion works for everyone. You're not asking someone to solve the problem, just offer insight which you are free to accept, reject or alter. The more opinions you collect, the more insights you may gather. By deliberating seeking the viewpoints, you are (hopefully) keeping an open mind - which is better for those half-mast (rhymes with..) days.