How so? If Battler unseals the tape, exits, then reseals it from outside, the chain is still set, still taped, and still prevents entry or exit. You can unlock a door, exit, and lock it behind you for a lock to still be intact. When Erika leaves looking under the bed and ready to grab Battler from the closet, the lock is still sealed. Just as it was when she closed the door when she first entered. She sealed it and locked it.
Battler unlocked it, left, and locked it again. Erika can't tell if it changed.
Well, she could if she checked the seal after checking the bed or if she had the Detective's Authority, but she never did. It became all about Kanon and how he wasn't in the closet. All smoke and mirrors.