I think that is normal for it to evaporate like that. The molecules are so small they slip right thru a tightly closed cap.
At the TV station we used alcohol. But the field crews used a cleaning tape which worked well. You are only supposed to play a cleaning tape about 10 seconds or so. If that doesn't fix it, you've got other problems.
I remember a CE I worked for once, looking at the alcohol bottle in the tape room, saying, "I think somebody is drinking this stuff," as we used quite a bit of it.
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