The 'iconic' civil ensign was used almost exlusively on civilian vessels as well as being flown at Austro-Hungarian consulates abroad, you'd struggle to encounter it outside these two cases however.

There do exist for example postcards with this flag on them although even those were fairly rare and were primarily intended for foreigners/tourists. All in all, the flag was used mainly abroad which is probably how it acquired its status in popular culture as 'the flag of Austria-Hungary'.

As to which flags you could've seen - the most official variation representing the monarchy as a whole was flying the Habsburg black-and-gold bicolour together with the Hungarian red-white-green tricolour and occasionally with the Imperial standard (Black Double-Headed Eagle in yellow field) as well.

In Cisleithania it was also common to fly the so-called Landesfarben ('Land Colours') that were specific to the crownland you were in, eg. in Bohemia you could encounter red-white bicolours either alone or flown together with the black-and-gold flags.

You could also see some more specific flags, particulary among nationalists like the Pan-German black-red-yellow tricolour or the Panslavic red-white-blue tricolour.