Hope Radar is feeling better. How long has the red discharge been present? Is he showing any other signs of illness? Sometimes this discharge can be present from issues such as a bedding change, new food reaction, build up of ammonia if you use a glass enclosure (it just doesn't "breathe" as well even if the cage is kept clean), or stress from a cage mate or a move of housing. It is not usually true blood (they can check under a UV light as porphyrin is flourscent under UV light).
We had this issue with our older male rat Kracker just last month. Tears and slightly reddish nasal discharge but nothing else apparent. It seemed to be related to a conflict between Kracker and Rocky our other male rat who is a bit younger and was being a bit aggressive to Kracker (dominance thing I think). We seperated them for a few days then gradually reintroduced them. Kracker sorted Rocky out about who is boss of the boy's cage. The red discharge disappeared and has not come back so we are pretty confident it was stress related.
Let us know how Radar is doing please.
~~Jo~~