sich etwas merken implies a conscious effort to retain something in your memory in order to be able to remember it in the future, so it equals remember, but only towards the future, not when it refers to remind something from the past (that would be sich erinnern) etwas merken or etwas bemerken refer to notice / realize but merken is only used with concrete or abstract nouns while bemerken can also be used with people (Hast du mich bemerkt?)

Thank you Joe!

Hi, they actually mean 2 different things. "Sich etwas merken" means "to remember something". So you're right about the reflexive form. "Ich merke" is somewhat tricky because it's deriving from the verb "bemerken". So "etwas bemerken" and the shorter version "etwas merken" both mean "to notice / to realize".