Yes, I think Officer French arrived on the scene first, but that didn't mean he was first to enter the basement. I cannot confirm my sources, and I expect neither can you. Either way, whoever arrived there first apparently noticed the lights were on.
Of course, it's pointless to discuss it, when neither of us know what actually happened.
Looks like we have another Officer looking into the basement that morning....Officer Reichenbach. Seems he arrived after Officer French did:
He (Reichenbach) went down into the sprawling basement and walked through it. At the far end was a white door secured at the top by a block of wood that pivoted on a screw. Reichenbach tried to open the door, stopped when he felt resistence, then returned upstairs. Reichenbach, Officer French, and one of the friends Patsy had called, Fleet White, would all check that white door in the basement during the morning, and White would even open it. They found nothing.
ITRMI ppb page 20