Interesting find.
Is it known whether the WW2 war-effort had any use for this area?
Because in that case there is a much increased likelihood of finding live ordnance there, e.g. 250kg aerial bombs. Even today such bombs are regularly found during construction work, they are typically deemed to unstable to move, so they are destroyed in place, a process that typically puts the construction (of the factory foundation) on hold for some days.
One would hope that Tesla has negotiated this point with the authorities, that the reaction to the discovery of any such bombs will be prompt and not cause a prolonged standstill at the site.
I don’t know about Germany, but in Belgium we routinely (daily) find bombs from WW1, and these are immediatly taken care of by a specialized division of the Belgian army. I doubt Germany will deal with this less efficiently. WWII bombs are a non-issue for GF4.
What is more risky is if the diggings would find things of archeological interest, as these could stop the works for months or years (at least that’q the case in Belgium)