How is inventory replenished from Requsitions, if there are no POs in the system?

I think your Material Management Department can answer this question. Usually you would have to look at where the source of your replenishment. If it comes from your vendor then you should have a PO. replenishment could also come from different location in your company or warehouse. If you are a retail establishment then some of the replenishment can also comes from returned items.

Look at the IC12 record for an item in question. Go to the Source tab and click on Replenishment. Here you can see if the items is replenished from a vendor or from another location within your organization. Click on Back and then click on the Reorder tab. The Reorder Point field will tell you how low the stock has to get before a purchase order will be generated (IC140) for vendor replenishment or a stock transfer document (IC141) for location replenishment. On hand quantity minus demand must be 1 less than reorder point to trigger an order.