With battery backups and various computer power supplies they can and do sometimes hold a residual charge. IN a UPS yes you need to remove the battery first. After that you can discharge it by attempting to power it up or plug in a load OR like metal said above me, leave it alone for a while. Most electrolytics cannot hold a charge for more than a few days. But some high end caps can hold a charge for weeks. Just take it easy, work smart, don't take apart anything that was just plugged in.
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