I agree with tarajanes. I'd put it under your pillow. Mine beeps pretty loud also and I still have a little 1 in my room and I DON'T WANT ANYTHING WAKING HER UP!!! Ha ha!! So, I just put it under my pillow or under the blanket until it stops beeping.
I have to aggree put it under your pillow,moving about will make a lot of difference in bbt....when you have finished taking your temp,try cupping the thermometer or put it back under the pillow,to muffle the sound.
I might try the under the mattress method. I just hope they don't hear the beep when the temp is done. I usually wouldn't worry that much, but it's the week I should start to see a spike in temp.
Is there a drawer by the bed that you can slip it into or maybe hide it under the mattress or pillow and then at 6 am while they are sleeping just take it out? They say not to move because it will change the temperature but I think that if it's just a few days it really won't make that much of a difference.