Presumably some people knew the real Quirrel person back when Voldermort was still alive. Maybe he just naturally had a bald patch all along.
Also, we don’t know that hiding a bald spot is possible (or safe). For instance, making someone pretty using magic is known to be very dangerous.
...in which case the bald patch isn’t evidence to us that Voldemort is or was there.
Presumably some people knew the real Quirrel person back when Voldermort was still alive. Maybe he just naturally had a bald patch all along.
Also, we don’t know that hiding a bald spot is possible (or safe). For instance, making someone pretty using magic is known to be very dangerous.
...in which case the bald patch isn’t evidence to us that Voldemort is or was there.