Apparently that note Terry left for Seamus Finnigan last week gave him enough information to work out that Terry is an animagus, and that probably there are others, likely including one that's small enough to have gone along to the rite.

Mind you, they already knew we had a spy at the rite; it's possible Bellatrix had already guessed there was an animagus involved. Rachel's warned Ron they might be on the lookout for animals, and she tracked Terry down tonight and had a chat with him. She said she was as gentle as she could be, but he got extremely upset with himself, and now she's worried she should have let you handle it instead. He's not equipped with tremendous self-confidence as it stands, and it's not as if he reports to her.

Frankly, I think it was inevitable they'd realise we had more animagi than just Sirius, sooner or later.