The verb いる (“to exist” or “to be”) requires a subject and optionally accepts a location.
In the case of Japanese such elements can be omitted in the surface sentence but they are inferred. So understanding this sentence requires understanding what to fill in those two slots.
In this case, subject = He/She/It (aka クロコ/Croco) , location = Here
So “He was here”.
“Just” is also sort of implied, so “He was just here”.
Btw if you the read the next sentence 怪しいやつが, that’s actually providing the subject for the first sentence (it’s inverted, aka a piece of grammar called 倒置法).
So you could say
“He was just here! That suspicious guy!” (maintain the inversion) or “That suspicious guy was just here!” (collapse the inversion).
And finally, in English it’s probably slightly more common to say “I just saw that suspicious guy”, or probably better for this context, “That suspicious guy! I just saw him!!” or something like that. The best translation (including whether you use “a” or “that”) would depend on the exact context which I don’t remember (it’s been a good two decades since I’ve played this game :)).