Stuck fermentation issue

On Sunday I pitched yeast into wort that read 1.08 OG. It was dry yeast, and I stupidly only used one packet. On Monday there was slight activity in the airlock, which then stopped. Knowing airlock activity is not reliable as a test of fermentation, I didn't worry about it. Then this week I realized I way underpitched. Last night I opened the bucket, saw very little sign of krausen, and sealed it back up with the intention of doing a hydrometer test tonight. (It's at my girlfriend's place so I didn't have the hydrometer when I stopped by.) I gave the bucket a couple of good shakes before I left to try to stir up the yeast. This afternoon, I had some free time to stop by and check on it, and it is bubbling very well. There was no off scent in the bubbles.

So, fermentation unstuck. Now, my concern is that it sat without fermentation for 5 days. Should I be concerned with infection or is there anything I should do to ensure the beer is ok?

As long as you kept things sanitary and didn't introduce anything into the brew when you opened the lid you should be fine.

CO2 is heavier than air, basically covering the brew with a blanket of CO2 initially pushing all the air out of the lock. The fact that you did have some activity in your airlock on Monday you should have had a nice layer overtop the beer.

If it's fermenting now, it's fine. You can take a sample to check the gravity and taste it if you really want, but I doubt there's anything to worry about.

Unless you spit in it when you opened the lid earlier, then that's a different story...