The human eye is remarkably bad for determining the altitude of an aircraft, drone or bird. The minimum safe altitude is either 500 ft or 1,000 ft depending on the density of the area. Except during takeoff and landing.
The FAA can't give permission for anyone to fly below the minimum safe altitude. If you are using ATC services (I.E. Controlled) then the radar is not expected to work below the MSA . If a pilot is communicating with Approach or Departure control and you ask for an altitude below their sector floor the response will be "Radar service terminated. Squawk VFR" and you will then be invisible to ATC.
No way was the airline aircraft that low if it's not taking off or landing, it just isn't going to happen:
14 CFR §91.119 Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a)Anywhere.An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
A 737 has a glide ratio of 17:1, so from 250 ft the glide will cover 4,250 ft before it's on the ground. Totally not legal. There's just no way that's going to happen on purpose.
You don't list your location in your profile, so tell me what airport the airline is flying out of and I'll see what the approach charts show.