Ignis was kidnapped by someone (thugs, drug lords, whoever it may be) and held for ransom for a bit.

Gladio would go out and search for Ignis with no luck, even with Prompto and Noctis helping and the Crowmsguard searching as well

But one day as Gladio is sitting in the garden by the front gates, he sees someone in the distance and lo and behold, it's Ignis

+His clothes are dirty and torn

++Ignis's wrists are chafed due to being toed with zipties

+++His feet are cracked and bloody from being forced to walk all the way back home by his captors