The first thing you mentioned, mumbling...my husband and AS son do that all the time. Makes them look psycho. (but they're not...I hope!)
The second thing, if you're talking about something more than daydreaming, yes. It used to happen to me very, very frequently, in any crowded or busy place (and particularly at school). It was somewhat like being shellshocked. I think it was sensory overload. It's just very weird and unnerving "coming to". A real jolt! I think it is the body's inabilty to simultaneously process everything that's going on. When I was very, very young (4-7) this used to occur even outside in the yard, field or woods. (I used to wander alot) I can only assume that even the sound of footsteps, crunching leaves, water, wind and the smells of the farmyard also were too much to process; though, I found nature calming.
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"Honey, would you buy me some boobles for my 40th b-day?" "No way, they're too expensive. Your own baubles will have to do."