shadows appearing randomly

I do a zoom where all participants cameras are off. Mine is on, yet covered, with a static jpeg background up. 
Then last week I randomly got these shadow images appearing. I couldn’t figure out if it was a person or what (see images). The shadows kept reappearing throughout the meeting. 
I thought it weird that one person in the zoom stats signed in and out five times during the 45 minutes. 
any thoughts on why the shadows happened? 
thanks