This is a custom proxy for the Grid, which will ensure that all the unattended alerts are cleared before the browser is recycled

MyProxy

package com.test.proxy;

import java.net.URL;

import org.apache.http.HttpHost;
import org.apache.http.HttpRequest;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.message.BasicHttpRequest;
import org.openqa.grid.common.RegistrationRequest;
import org.openqa.grid.common.exception.GridException;
import org.openqa.grid.internal.ExternalSessionKey;
import org.openqa.grid.internal.Registry;
import org.openqa.grid.internal.TestSession;
import org.openqa.grid.internal.TestSlot;
import org.openqa.grid.selenium.proxy.DefaultRemoteProxy;

public class MyProxy extends DefaultRemoteProxy {
	private final int MAX_NETWORK_LATENCY = 1000;

	public MyProxy(RegistrationRequest arg0, Registry arg1) {
		super(arg0, arg1);
	}

	public void beforeRelease(TestSession session) {
		System.out.println(">>>>>>> Session is about to be released");
		boolean moreAlertsPresent = true;
		do {
			moreAlertsPresent = dismissAlerts(session);
			System.out.println(">>>>>>> Processing all stale alerts");
		} while (moreAlertsPresent);

		super.beforeRelease(session);
	}

	public boolean dismissAlerts(TestSession session) {
		TestSlot slot = session.getSlot();
		ExternalSessionKey externalKey = session.getExternalKey();
		URL remoteURL = slot.getRemoteURL();

		HttpRequest request;
		switch (slot.getProtocol()) {
//code borrowed from DefaultRemoteProxy.java. Not sure how the URL would be for Selenium protocol.
//Can conk when used with Selenium protocol.
		case Selenium:
			request = new BasicHttpRequest("POST", remoteURL.toExternalForm() + "/?cmd=testComplete&sessionId="
			        + externalKey.getKey());
			break;
		case WebDriver:
			String uri = remoteURL.toString() + "/session/" + externalKey + "/dismiss_alert";
			request = new BasicHttpRequest("POST", uri);
			break;
		default:
			throw new GridException("Error, protocol not implemented.");
		}

		HttpHost host = new HttpHost(remoteURL.getHost(), remoteURL.getPort());

		boolean ok;
		try {
			HttpClient client = getClient(slot);
			HttpResponse response = client.execute(host, request);
			int code = response.getStatusLine().getStatusCode();
			String statusMsg = response.getStatusLine().getReasonPhrase();
			ok = (code == 204);
			System.out.println("Return code for alert processing = " + code + " reason = " + statusMsg);
		} catch (Throwable e) {
			ok = false;
			System.out.println("Error releasing. Server corrupted ?");
		}
		return ok;
	}
	
	 private HttpClient getClient(TestSlot slot) {
		    Registry reg = slot.getProxy().getRegistry();
		    int browserTimeout = reg.getConfiguration().getBrowserTimeout();
		    if (browserTimeout > 0){
		      final int selenium_server_cleanup_cycle = browserTimeout / 10;
		      browserTimeout += (selenium_server_cleanup_cycle + MAX_NETWORK_LATENCY);
		      browserTimeout *=2; // Lets not let this happen too often
		    }
		    return slot.getProxy().getHttpClientFactory().getGridHttpClient(browserTimeout);
		  }
}

it needs a org.openqa.grid.internal.Registry which is for 2.8