Kath, perhaps if/when you notice she has gone off & hidden, you could then ask her if she needs to do poos & try to catch her before she's actually done it & get her to sit on toilet or potty.

Is there a regular time that she poos? If so maybe get her to sit on the toilet or potty infront of the TV or something & try to catch it so to speak & praise her for it, make that a regular thing, to sit on the potty or toilet at the same time, until it all clicks...
If she is comfortable to do wees on potty, maybe get her to go back on potty until she is doing both wees & poos on potty...?

Maybe her doing wees on toilet is making her scared to do poos on toilet & so she is starting to wet?

Good Luck, hope that helps!!!