Where does the + 3 dB come from in active radar detections?

[JuergHuber]

There is an additional + 3dB that I don't understand in the monostatic radar detection code:

openburst/openburst/functions/radfunctions.py

Line 306 in b1e9e23

alpha = pow(10, ((snr + 3) / 20))

I understand that the Bayes test is used for detection as mentioned in this paper (Equ. 18-23).

I understand that the SNR is calculated according to Skolnik 1980, (Equ. 2.54) (apart from the bug in #15).

It is unclear where the additional + 3dB term comes from.