To sweep a enemy minefield, you send the mine sweeper ship to it. It will automatically sweep mines, until the sweeper components are saturated, then the ships will take damage.
If I understand you correctly, you want to sweep a known ally minefield to aid someone else. The rules do not allow that.
I wonder, where else has your ally put mines, orbiting your planets perhaps? Oh well, break your treaty and find out. Bwa ha ha ha.
[EDIT] Oh wait, I read your post backward. If you've gone to a known enemy minefield, and you didn't sweep anything, and your ships didn't take damage, then all mines were consumed in previous attacks that completely destroyed ships. The mine field indicator does not fade automatically. Use the sweep mines button to insure it does. Decloaking should not be necessicary.
[ October 23, 2002, 22:07: Message edited by: Arkcon ]