Originally Posted by SLF
Spooky! The B- registration prefix is used by China, Hong Kong, Taiwan, Bulgaria, Albania plus a few others - Belgium not being one of them (O- or OO-). Perhaps a cargo plane, being transferred between airlines? Or you, I and this thread will disappear shortly

I googled it and found that registration codes are unique amongst countries. No countries share a code. Albania is
ZA, Bulgaria is
LZ, Belgium is
OO. China is
B. Hong Kong and Taiwan also use
B as they are not listed as separate countries. I guess whoever responsible for the list subscribes to the one China policy.
The aircraft OP saw must have been a freighter registered in China, Taiwan or Hong Kong. My guess is that it is a recently acquired freighter with the old paint scheme stripped, but without the new paint scheme being applied. Maybe it was being ferried to China?