True, the balloons themselves don't.

But their background does, and when they move, this causes changes in the IR signature/profile from the ambient background, So, you get activations.

Same thing can happen when a HVAC quickly heats or cools the lens and/or PIR housing.

Any change, or "perceived" change on the part of the passive IR sensor caused be an intermediate object between the actual PIR sensor and the ambient background can result in a trigger. Balloons can fit this criteria.

All due respect, MNSHO, YMMV, qualified professionals may disagree, etc. etc.

Learning as fast as I can.