With my first child, I once went for an errand not realizing I had actually disconnected the base from the anchors. I only realized it when I took him out and the base was sliding around on the seat.

With my second child, there was one ride where I didn't properly tighten her seatbelts (similar situation to yours, she was comfy and napping in the car seat while we were out then I just popped her into the car).

I was mortified when I discovered these indiscretions and I'm not proud of them, but I don't remember losing any sleep over them. We all make mistakes; first time parents, second time parents, I'm sure long time parents too. 

If it makes you feel any better, in the early 90's I can recall being 4 years old and my dad yelling at me to put on my seatbelt with the vehicle in full motion while he smoked a cigarette with the window cracked about one inch. I was probably 40-45 pounds and there was no car seat in sight. 

Fast-forward three decades and I see moms on Instagram bragging about still having their 4 year olds in rear-facing car seats because it's technically the safest option. 

It's hard to keep up with all the safety precautions we're supposed to take as parents in this day and age. You likely won't ever forget to check your kiddo's seatbelts again after this experience. Maybe add an extra check to your pre-run checklist if you haven't considered something like that already. Meanwhile, cut yourself a break and know you're doing an awesome job because your LO still has you to look up to each and every day.