Я что-то тоже большой лексической разницы не вижу.
Разве только secure иногда перекликается с guarded.
Safe - adj. Secure from danger, harm, or evil.
c.1300, "unscathed, unhurt, uninjured; free from danger or molestation,
in safety, secure; saved spiritually, redeemed, not damned;" from Old
French sauf "protected, watched-over; assured of salvation," from Latin salvus "uninjured, in good health, safe," related to salus "good health," saluber "healthful," all from PIE *solwos from root *sol- "whole" (cognates: Latin solidus "solid," Sanskrit sarvah "uninjured, intact, whole," Avestan haurva- "uninjured, intact," Old Persian haruva-, Greek holos "whole").
Secure - adj. Free from danger or attack: a secure fortress.
1530s, "without care, dreading no evil," from Latin securus,
of persons, "free from care, quiet, easy," also in a bad sense,
"careless, reckless;" of things, "tranquil; free from danger, safe,"
from *se cura, from se "free from" (see secret (n.)) + cura "care" (see cure (n.)).
In English, of places, "free from danger, unexposed," from 1580s.
Meaning "firmly fixed" (of material things) is from 1841, on notion of
"affording grounds for confidence." Of telephones, "not wiretapped,"
from 1961. Replaced Middle English siker, from Old English sicor, from the Latin word.