The decision by the D.C. Board of Elections and Ethics that Human Rights Director James W. Baldwin did not violate the city's conflict of interest code - as Baldwin had already acknowledged he did - poses an interesting legal riddle.

If the board had enough evidence to formally charge Baldwin (as it did in April 1977) and if the evidence was strong enough to compel Baldwin to plead guilty (as he did in July 1977), how did the board's hearing examiner conclude 10 months later "that there is no evidence" on which to find that Baldwin violated the law?

Three little words appearing in the five-page decision handed down last week by board member and hearing examiner Jeanus B. Parks Jr. are the answer. The words are "in THIS record." They are a protruding indication that the findings of fact supporting Park's decision - similar to the findings of other recent city hall investigations - are limited findings based on only some of the facts.

The allegations against Baldwin stem from his authorship of two letters to about 80 city employes attempting to recruit them as students for a graduate program in public administration offered by Nova University of Fort Lauderdale, Fla. Samuel Humes, director of graduate studies at the school, said that at the time Baldwin sent out the letters, he was "in line" to become director of the student group and, if hired, could have been paid up to $500 per month if enough students signed up for the classes.

The alleged conflict of interest was that Baldwin was using public resources in an way that could possibly mean private gain - his own. At a hearing July 26, Baldwin acknowledged sending out the letters, but said he never received any money from Nova. Although he may have technically violated the law, he said, he did not intend to do so.

Parks reached his decision without questioning Humes, who said in an interview this week that he would have been glad to tell his story under oath if some one had asked. But no one did.

Humes said he did have a telephone conversation with someone whose name he could not recall. That someone, according to other sources, was a former staff lawyer for the ethics board. Park's decision gives no indication that Humes' story, as told to the lawyer or anyone else, entered into his deliberations. Parks simply concluded that there was no evidence "in this record" to indicate an effort by Baldwin to realize personal gain. Of course not.

Whatever reasoning underlies the procedures in the case will not be clear until later, if even then. Parks is not returning reporters' telephone calls and board Chairman Shari B. Kharasch, along with general counsel Winfred Mundle, will not talk about the decision until they are assured that it will not be appealed.

The nagging thing about the Parks decision is not so much that it can be viewed as letting Baldwin off the hook.He has already pleaded guilty, and there may be only questionable value to hitting a man when he's down.

Rather, the concern is that the investigative process, which is supposed to impartially resolve the issues at hand, leaves key questions unanswered - because they were apparently unasked. Perhaps more importantly, it is similar enough to the findings of other such "investigations" coming out of city hall over the past two years to suggest that a pattern is developing.

When City Administrator Julian Dugas was assigned to investigate possible nepotism and cronysm in hiring on the part of former Human Resources Director Joseph P. Yeldell, Dugas never questioned Yeldell. In another case, when Municipal Auditor David Legge was told that Yeldell had sought help from businessman Dominic F. Antonelli Jr. in getting two personal loans, Legge didn't bother to include that information in his report. Yeldell and Antonelli were subsequently indicted by a federal grand jury on bribery and conspiracy charges stemming from the loans.

When the city's campaign finance office was investigating allegations of illegal cash contributions in Mayor Walter E. Washington's 1974 campaign, the investigators restricted their probe to a time period that did not coincide with the time the allegations were said to have occurred. As a result, many of those with first hand information about the alleged cash payments who were called never divulged the information because they were not asked.

When reporters asked Police Chief Burtell W. Jefferson recently if he planned to interview a senior police official who reviewed police promotional exams about allegations of irregualrities in the administration of those exams, Jefferson said no. To ask those kinds of questions of a senior police officials, Jefferson said, would be to impugn that's official's integrity.

The effect of such narrow investigations seems to be counterproductive. Instead of resolving the questiones, which might be the only sure way to make them go away, the uncertainties linger on.