I don't know whether "cache" is actually from the same root as "kaŝi", but it's sure a good mnemonic!
cache (n.) 1797, "hiding place," from French Canadian trappers' slang, "hiding place for stores" (1660s), a back-formation from French cacher "to hide, conceal" (13c., Old French cachier), from Vulgar Latin *coacticare "store up, collect, compress," frequentative of Latin coactare "constrain," from coactus, past participle of cogere "to collect" (see cogent). Sense extended by 1830s to "anything stored in a hiding place."
I anyone else unable to understand the first letter on this sentence?
No. It would either be "Li kaŝis sin" or "Li kaŝiĝis." The verb "kaŝi" takes a direct object (in this case "sin") but the suffix "iĝi" turns "kaŝi" into an intransitive verb which cannot take a direct object., so "kaŝiĝi" acts on the subject all by itself.