Share 'weird magnetic field?'

Every time I lock up near Clark and  Washington I notice that my wireless bike computer thinks it is travelling fast, like 30, 60 or even 90 mph. The numbers move around as I pick up the device to take it off my bike. The device still thinks it is moving  fast after it is in my hand and no longer  on the bike.  (I bring all loose items such as lights, computer etc, in with me.) My trip from home…