Just a Victoria secret bra RFID tag for inventory purposes. This type of antenna coils have very long range. Glass implants because their size have way less range and need a specific angle to be read. So if you are still on the idea of RFID tag for the sponge, you will need to consider the angle and distance that the tag/reader combo should be set up.
I like technical solutions, but this is probably a keep it simple situation
Attach one end outside of sink, attach one side to sponge
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Yeah… My thought exactly: a RFID transponder and a wireless alarm to keep a sponge put is really overthinking it.
Alternative solution: drill a very small hole in the sink’s bunghole - small enough to fill the sink and do the dishes normally, but letting the water drain out over a few hours if the sink is left full of water. No water, no floating sponge 
If you want an electronic solution, maybe RFID is overkill. How about magnets and a reed switch? I take it you’re not too concerned with identifying exactly which sponge is placed there, just that the one that’s out is in it’s home?