Can an mri always detect a stroke ~5 days after?

Probably: Especially if the MRI included a "diffusion-weighted imaging" sequence (which may or may not be performed in a standard MRI brain study.) http://emedicine.Medscape.Com/article/1155506-overview#a1.

Mostly yes: An MRI is a a very sensitive test that (if we disregard image degradation from motion etc) should be able to detect even small stroke even a few hours after onset. The appearance of stroke changes with time but they can be detected with a fairly high degree of certainty nevertheless. The history of the patient however does play a role as sometimes e.G old traumatic lesions can have a similar look.