Cadsuane and the WO aren't the only ones who could deduce Cadsuane kept the domination band and where. She intended to test it eventually on an Asha'man, but still had to convince Rand to let her (she had made another attempt in chapter one). Her followers and probably pretty much everyone present at Semirhage's capture probably knew about that (it's when Cadsuane grabbed the objects...). Cadsuane had promised Rand it was safe.

Most importantly, notice that at the first meeting with Cadsuane upon their arrival, Bair and Sorilea already knew Cadsuane had male/female a'dam and took the initiative to ask her about that. Cadsuane wasn't in any way surprised that they knew. They knew she was the one keeping them. So, it wasn't a secret that Cadsuane had the DM.

Her hiding place wasn't quite "clever": in her room, in a trunk. Riiiight. No one would search her room first, no one would look in her trunk after trying her jewels box. Cadsuane sinned by overconfidence. She thinks she has everyone under control, and that inverted weaves can't be detected. She was wrong.

The other obvious fact: Cadsuane has not even wondered (nor Nynaeve, nor the WO) how the Shadow could find the DM. There was nohunt for the DF who led Elza to it, for the good reason everyone knew Elza needed no one telling her Cadsuane had the DM.

We also know it's Shaidar Haran who made Elza steal the DB. It's probably him who could detect wards. Most likely he asked Elza in the first place where the a'dam taken from Semirhage were. Elza answered "probably in Cadsuane's room". SH made her search it, they found the warded trunk, and SH taught Elza how to unravel the ward (yes, she probably used the TP for that. SH couldn't do it itself because of limitations to his interaction with the physical world).