Yes this can happen even without running the bike. Temperature changes from night to day can be enough to cause the gas in the tank to contract and expand. There is a pressure relief valve under the gas cap, its just a small rubbery thing and when dirt, wax and stuff get it dirty it will squeal instead of working quietly. Take a can of compressed air and spray around the underside of the gas cap and this will often get rid of the problem.
This happens to my bike whenever I overfill the tank.
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