no way was it stolen, you need to jack the car to get enough access to get the front bolts, and to get to the rear bolts you'd need to be able to turn the wheel. and even then its almost an impossible job to reach, it really has to be a wheel off job to get it out.
i reckon your garage forgot to put it back on.
also if they jacked the car up, which they would have had to, it would have set the alarm off, most modern car alarms are designed to go off if they change angle. i.e. it shoul dhappen if your towed, or jacked, the idea being to stop people stealing wheels