Ran across this line ㊙︎DVD, with the 秘 encircled. Do any of you know what this might mean?
This is a common abbreviation style to suggest a certain phrase by enclosing a single kanji in the word. Some frequently used symbols are now officially available on the computer (see 1, 2) due to typographical needs.
In this case,
㊙︎ = 秘密【ひみつ】 = "secret, confidential, classified" (often in ads or catch lines)
It can be read aloud 丸秘【まるひ】 "circle(d) ひ". The formal typographical name is 丸囲み【まるかこみ】秘. This character can also be used as an emoji (㊙️).
There are unencoded but famous ones including マル暴, which is originally a police jargon associated with 暴力団【ぼうりょくだん】 or yakuza.