I think we all love to hate Harbor Freight, but you can score some great deals on useful things. To whit:
This shows up on the Harbor Freight site under 3 different catalog numbers, but they appear to all be the same product. It isn't specified how far away or in what arc it can "see motion", but the unit uses 433 MHz to transmit to it's base with a quoted range of up to 400'. You'll need 1 9v and 3 C cell batteries, which means in a grid down scenario you'll need a lot of spare lithium ion batteries and ways to charge them.
Cheap way to stretch your sentries.