Why have I had a dent suddenly appear in brow? (Photo)

Hi! Without an arrow pointing at exactly where you think you are seeing this irregularity, then I would say generally, yes this is normal. You are likely seeing the bulk/action of the procerus muscle and/or the corrugator supercilii muscles. You could consider doing neurotox to these areas to try to relax the muscles and hopefully with time make them smaller so that there is less of a discrepancy.

This is very likely due to interplay of underlying muscles and thinning subcutaneous fat tissue? A small amount of diluted filler would help.