2003 Nissan Altima oil mystery

I took my Nissan in to get the oil changed
and was told that there was a pretty bad
leak. Took it to a local mechanic and had
the valve gasket cover replaced along with
a few other things like spark plugs and tire
rods. Almost 3 weeks later I notice a puff of
white smoke coming out of the back of the
car when I crank it after it’s been sitting all
day. I checked my oil the next morning and
it’s bone dry. Get the oil filled again and
take it in. They say it’s not leaking but
maybe burning oil so keep a check on it
the next few weeks. It’s been a week and a
half since I got oil put back in and my oil is
still full. The only thing I can think is that
the mechanic did not put oil back in after
doing the repair like they were supposed
to. Are there any other possible
explanations?