Sure you can have lower fermenter temps vs ambient. You need fluctuating ambient temps that warm up from a cooler temp.
Generally early on in the process you have fermentation temps that are higher than ambient due to yeast generating heat as they work. After most of the fermentation is done, they are no long contributing any heat to the beer, so you see fermentation temps trending with ambient (with time lag of course).
I would strongly suggest a hydrometer reading if you really want to make sure this beer finished. Early on in your homebrewing, its a good idea to take readings a few days apart and make sure they don't change (i.e. your beer is finished with no risk of bottle bombs). Then make sure it got enough time to clean up byproducts. Eventually this will all be second nature to you, and you can just take one reading prior to packaging.
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