enterprise car rental Charged extra days after car was returned via night dropoff Austin Texas

I rented for weekend special rate, with arrangements to return Sunday after their 3 pm close.  They did not succeed in upselling me.  Nor did they catch me in the fine print to add charges.  

Returned car Sunday evening in center of well-lit lot and placed keys in secure drop box as instructed. Witness verified.  Drove my own car Monday, also witness verified. 

Monday they said they had no keys and no car so rental charges would continue at highest retail daily rate. manager suggested I file auto theft report with my insurance company and police.

Filed auto theft report. Tuesday mid-day they told police that the car was located on their lot and keys in dropbox so no theft.  The manager says I will be charged two additional days instead of any refund of my security deposit.

Regional Enterprise office does not return my calls.