The metaphoric use of the term radar is in general use. It refers to the set of things that some entity is paying attention to. (That entity being the owner of the radar.)
- To fall off the radar is to stop attracting attention or garnering media coverage.
- Something that flies below/under the radar is not being detected. This could be intentional (you may be trying to elude detection) or unintentional (you want to be noticed but can't get anybody to pay attention).
- If you say that something is not on your radar, it means that it is not something you have been paying attention to.
- To put on someone's radar is to draw someone's attention to something. If you put something on your own radar, it means that you are making an effort to pay attention to it.
At Microsoft, the metaphorical radar usually belongs to a specific person or group. You might forward an email exchange to a person or group with the message "Just wanted to make sure XYZ was on your radar." In other words, "Just wanted to make sure you were aware of XYZ." There is no requirement that the recipient take action in response to the message, but you're letting them know about XYZ, just in case they weren't already aware of it.
You can also "put something on X's radar", which means the same as in general use: To make X aware of something. Although there is no requirement that the recipient take action, the phrase is usually used when you expect that X do something. For example, "We're going to be removing support for the beta XYZ interface, and everybody needs to switch over to the RTM interface by next month. Can you put this on the radar of our partner teams?" Translation: "Can you make sure that our partner teams are aware of this? I don't want them to complain that we never warned them."