Peugeot 308 loss of electrical power when alarm set.

I purchased this car three weeks ago and was told the oil was checked prior to me driving away.
Two days ago, I put the key in the ignition and the car just lost all electrical power. I opened the bonnet to find the oil filler cap missing and no oil showing on the dip stick. I topped up the oil and after a few minutes the car started as normal.
The following day, the car started fine and got me to work. On getting ready to leave for home I unlocked the car with the remote fob. The alarm went off and there was a total lack of electrical power again.
I "wiggled" the panel connected to the negative terminal of the battery and the alarm got louder and I was able to start the car and the alarm stopped.
The following day (today) the car ran fine in the morning but the alarm went off spontaneously with a total lack of electrical power. Again i wiggled the panel and I was able to start the car.
Does anybody have any pointers as to what is wrong with this panel. I believe that there is a cover missing. Also I don't believe that this panel should be loose enough to "wiggle".
All the terminals look and feel secure.
Thanks in advance.