Are there bridge behaviors that are technically within the rules that could be considered unethical?
First: I'm speaking sanctioned duplicate bridge.  What happens in the rubber clubs is out of my experience; the Laws are different, and obviously when money is on the line, different things are ethical or unethical.

To answer your title: yes.  Because the ethics of the game are fully defined by the Proprieties (which were moved from "guidelines" to actual Laws decades ago), and "considered unethical" is frequently in the eye of the beholder, there are many things that are entirely Proper that society, even bridge society, would side-eye you for.  I remember explaining to a newer player that misbids were legal, and in fact, deliberately misdescribing your hand was legal under certain conditions (for instance, as you say, that it truly is a misdescription to all three opponents, and not something partner "expects').  She said that lying is totally unethical.

The other common ones I get complaints about are Law 72B2: no obligation to draw attention to an infraction committed by one's own side [except mistaken explanation], and frustrations when people have got a good score via Law 10C4: [excepting use of UI] it is appropriate for the offenders to make any call or play advantageous to their side, even though they thereby appear to profit...

There are other behaviours that are definitely "borderline unethical" that don't cross the actual letter of the Proprieties.  Many of them are made illegal by regulation of the Bridge Organization running the game (for instance: ACBL's Zero Tolerance for Bad Behaviour policy, or the EBU's Better Bridge Program).  And it is unethical to deliberately violate a Lawfully created regulation of your NBO (Law 72A, Law 80B2f).

To answer your question specifically, the Law 40C1 says, in part: Repeated deviations lead to implicit understandings which then form part of the partnership’s methods and must be disclosed in accordance with the regulations governing disclosure of system.

As others have answered, tendencies of your partner (as opposed to tendencies of players in general, even flight A players who psych more than 0%) become implied agreements very quickly.  It doesn't take more than one or two times partner opening 1NT on KQTxxx of clubs and out (and passing Stayman) for you to be aware partner might be doing this.  Now that it is an implied agreement, it may be an illegal agreement in your bridge organization ("Comic 1NT", 15-17 or a 2-level preempt, is illegal in the ACBL for instance). If it is not illegal, if you do not explain your actual agreement (which includes "but partner has been known to have ... instead"), you are misinforming the opponents, which is also an infraction.

Also, many bridge organizations have provided regulations or guidelines on psyching.  The ACBL has this to say about psychic bids.

In particular it points out three things that would be concerning, and should be investigated:


Excessive Psychic Bidding — When three or more psychic initial actions by members of a partnership have been reported in any one session and are called to the attention of the Director, the Director should investigate the possibility that excessive psyching is taking place. [The onus is on the player to prove that it is not, in this case]


[Note what is considered excessive, though - two or three in 26 boards.  Most players who do psych make it to two or three in a year]


Frivolous Psychic Bidding — Any psychic action inspired by a spirit of malicious mischief or lack of will to win may be interpreted as frivolous.
Unsportsmanlike Psychic Bidding — Action apparently designed to give the opponents an abnormal opportunity to get a good score, psychs against pairs or teams in contention, psychs against inexperienced players and psychs used merely to create action at the table are examples of unsportsmanlike psychic bidding.


It also has the category, totally banned, of risk-free psychics; those that are protected by system in any way.  A classic example is, after an opening 10-12 NT, agreeing that partner's 2 of a suit (to play) MAY NOT BE raised by opener, even with AKQT.  If that is because you would ever bid 2 hearts with xx to pick off their suit, your agreement is illegal.