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Most voltmeters will only carry a few milliamps. You don't want to flame broil your VOM, so use the DC voltmeter setting to check for drains.
Pull one of the cables off your battery (preferably the negative, since negative should always come off first and go on last).
Between your battery terminal and your battery cable, you will likely read somewhere in the range of 4 to 8 volts. That is fine, and normal for a system with semiconductors in it. If you have an electrical drain that will drain your battery, you will read full battery voltage between your battery and its cable.
Start pulling fuses until the voltage drops down below battery voltage. The problem may well be a stuck relay as suggested. Could also be a light that is staying lit in the glove box or trunk, or any one of a number of other things. It could be a failed diode in the alternator, though they usually fail open rather than shorted.
The wild card is the clock. I don't know how Volvo clocks work. In the old days, car clocks just took an occasional burst of power, so you could check the rest of the dash circuit while the clock ran on its own. When the clock demanded power, you just touched the cable to the battery terminal, and the clock would wind and not demand power for several more minutes.
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My 85' 240 had a drained battery after 2 weeks of sitting...twice already with new battery [200][1985]