If no one would bump into it, you don't need the lights. If someone
might bump into it, you do need light. Yup - that's sound logical.

Capt. JG wrote:
Seems pretty straightforward...

(e) A vessel of less than 7 meters in length, when at anchor, not in or near
a narrow channel, fairway, anchorage, or where other vessels normally
navigate, shall not be required to exhibit the lights or shape prescribed in
paragraphs (a) and (b) of this Rule.

(a) A vessel at anchor shall exhibit where it can best be seen:

(i) in the fore part, an all-round white light or one ball;

(ii) at or near the stern and at a lower level than the light prescribed
in subparagraph (i), an all-round white light.

(b) A vessel of less than 50 meters in length may exhibit an all-round white
light where it can best be seen instead of the lights prescribed in
paragraph (a) of this Rule.