I'm sorry this happened to you.

If I'm worried about the contents I use coloured cable ties and always have an anchor point on the outside of the bag to protect the zippers.

Carry the spare cable ties in carry on to prevent re-sealing in case of theft. When you collect the bag you can instantly spot any tampering - not that I've ever seen any.

The worst attempted theft from mine was a slash of the bag through an (open, empty) outside pocket. Thief opened the pocket and cut through the lining behind the pocket flap, covering the theft with the closed pocket. Didn't spot this until I started to unpack at home. Nothing in the bag except dirty, scruffy clothes and a laptop cable. Nothing stolen!