They’re actually quite resilient especially if weather is humid. They don’t need to eat every night either. I’ve had slugs in houses where there was no visible food left for them, and they stayed. They do like scraping mold and rot off a variety of surfaces, so it’s most likely it survived on whatever was growing or rotting underneath the carpet, or behind a damp wooden skirting or windowsill. Stuff that us humans wouldn’t notice unless went literally looking for it