Tank sweep completed but can’t scan front porch

[Mina G.]

I just got a tank sweep done by a reputable company here in NJ. They scanned the sidewalk, backyard, and checked the basement and outside around the house for signs of oil piping. Nothing was found. This house was built in 1920 so it’s either they stuck with coal until gas lines were installed, they had above ground tank and they removed all the piping, or they removed the tank long time ago (town doesn’t have record of tank removal permit) or it’s somewhere under the front porch which is reinforced concrete porch. The company wasn’t able to scan it because of that. 

They said that if I want to get the porch scanned, I would need to hire another company that does radar scan (which is much more expensive)

How likely is it that the tank is under the concrete porch? 

Current furnace was installed in 1987 (very old) and it’s forced air furnace (no radiators)

Any recommendation on what I should do? Thanks! (Seller is selling the house as is)